1. Field of the Invention
The disclosures discussed herein relate to a developing device, an image forming apparatus, and a process cartridge.
2. Description of the Related Art
There is known in the related art a developing device employing a two-component developer composed of toner and magnetic carrier as a developer so as to develop a latent image formed on a latent image bearer to form a visible image. In such a developing device, the two-component developer contained inside a casing is supplied to a developer bearer, and the developer bearer supplies toner contained in the two-component developer on the surface of the developer bearer to a latent image on a latent image bearer in a developing area facing the latent image bearer to visualize the latent image.
In the developing device employing the two-component developer, since the toner contained in the developer inside the casing is consumed by development, toner is supplied by a toner supply device. In order to maintain a developing ability of the developing device, toner needs to be appropriately supplied to the developer in the developing device such that a mixing ratio [wt %] of toner to magnetic carrier in the developer used for the development falls within a predetermined range. As an example of a toner density sensor to detect toner density of the developer, Japanese Laid-open Patent Publication No. 2012-108483 (hereinafter referred to as “Patent Document 1”) discloses a technology to detect toner density utilizing magnetic permeability of the developer that varies with the toner density. In the output of the toner density sensor utilizing the magnetic permeability change of the developer, when the toner density is low, the carrier charge near the toner density sensor increases to raise the magnetic permeability of the developer, thereby increasing an output value of the toner density sensor. By contrast, when the toner density is high, the carrier charge near the toner density sensor decreases to lower the magnetic permeability of the developer, thereby decreasing the output value of the toner density sensor. Hence, the toner density is detected based on the output value of the toner density sensor, and output characteristics indicating a relationship between the output value of the toner density sensor and toner density.
In the technology disclosed, for example, in Patent Document 1, when the developer is stirred at a stirring speed other than the standard stirring speed, the output value of the toner density sensor is corrected to obtain the output characteristics (the relationship between the output value of the toner density sensor and toner density) when the developer is stirred at the standard stirring speed.
The bulk density of the developer fluctuates with the carrier charge. That is, when the carrier charge is low, electrostatic repulsive force between carrier particles is reduced. Hence, particles of the developer become more tightly packed to increase the bulk density. On the other hand, when the carrier charge is high, electrostatic repulsive force between carrier particles is raised. Hence, the bulk density of the developer is lowered. The carrier charge may, for example, fluctuate with humidity. The carrier is more susceptible to charge, and the carrier charge increases as the humidity decreases.
The disclosed related art technology handles the output characteristics at the standard stirring speed as an unchangeable factor, and hence, uses predetermined output characteristics. However, the susceptibility of the carrier to being charged may vary, for example, with humidity. Hence, at the standard stirring speed, the developers having the same toner density may have different bulk densities due to conditions such as humidity. That is, the output characteristics at the standard stirring speed may vary with humidity. Hence, in the related art technology, the toner density at the standard stirring speed obtained based on the predetermined output characteristics and the output value of the toner density sensor may differ from the actual toner density due to the conditions such as the environment. Accordingly, the related art technology may fail to detect the accurate toner density due to the environment and the like at the standard stirring speed. As a result, the density of toner in the developer inside the casing may fail to be controlled, thereby obtaining image density failure.